Method of storing a data bit including melting and cooling a volume of alloy therein

ABSTRACT

Alloy memory structures and methods are disclosed wherein a layer or volume of alloy material changes conductivity subsequent to introduction of a electron beam current-induced change in phase of the alloy, the conductivity change being detected using current detection means such as photon-emitting P-N junctions, and being associated with a change in data bit memory state.

BACKGROUND OF THE INVENTION

The predominant mass storage device in conventional computing devices isthe hard disk drive. Hard disk drives are relatively large,electromechanical devices that can store a relatively large amount ofdata. The stored data is accessed through a read/write head that rideson a cushion of air above the rapidly rotating disk. The read/write headmoves radially to access data in different tracks of the rotating disk.Data transfer is limited by the speed at which the disc rotates and thespeed with which the read/write head is positioned over the requiredtrack. Even with the fastest devices, access times are on the order ofthousands of microseconds, due to the relatively large mechanicalmotions and inertia involved. This time scale is at least seven ordersof magnitude slower than the sub-nanosecond time scales at whichprocessors operate. The discrepancy may leave the processor starved fordata. Compact Disc and DVD storage systems, also limited by the speed atwhich a disc rotates and the speed with which a read/write head ispositioned over a required track, are associated with similardiscrepancies.

During the time the processor is starved for data, either valuablecomputing time is lost or the processor must perform another task, whichalso may lead to data starvation. Such data starved conditions arereferred to in the art as being input/output (I/O) bound orbottlenecked.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited in the figures of the accompanying drawings, in which likereferences indicate similar elements. Features shown in the drawings arenot intended to be drawn to scale, nor are they intended to be shown inprecise positional relationship.

FIG. 1 illustrates an alloy memory element.

FIG. 2 illustrates an alloy/LED configuration.

FIG. 3 illustrates an alloy/LED memory device.

FIG. 4 illustrates an alternative embodiment of an alloy/LED memorydevice.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements, and in which is shown by way of illustration,specific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. The following detailed descriptionis, therefore, not to be taken in a limiting sense, and the scope of theinvention is defined only by the appended claims.

Devices and methods are disclosed to store data using a memory withoutmoving parts. In one embodiment of the present invention, an electronbeam (e-beam) is used to irradiate a volume or layer of an alloymaterial which corresponds to an encoded data bit location. Exposure ofthe alloy material to a modulated high-energy e-beam changes theconductivity of the alloy material, creating at least two differentstates of conductivity, which may be associated with the binary data bitmemory states of, e.g., “0” and “1”, or any multilevel value between thetwo terminal states. Reading the stored data is accomplished with alower energy e-beam that does not alter the state of conductivitycreated by the high-energy e-beam during data writing. The lower energye-beam may be used according to various embodiments of the invention toread the conductivity of the alloy material, which corresponds to thedata stored therein. The invention is not limited to storing and readingbinary data. The invention is applicable to n-ary data, however tosimplify the description, the example of binary data will be followed.Mass-related issues with moving parts, such as inertia, have previouslylimited the data seek time in conventional devices, which is associatedwith rotational latency and mechanical steering of a read/write head.

The association of the binary values for data bit memory states, e.g.,“0” or “1”, is completely arbitrary with respect to the state ofconductivity within the alloy; the invention is not limited by theassociation made. An extension to n-ary data may be achieved by creatingmore than two distinct conductivity states within phase change material.For example, three distinct conductivity states could be associated withtrinary data.

With reference to FIG. 1, device 100 includes a volume of alloy 110coupled with a conductor 112 and a conductor 114. A variable-energye-beam 120 operating at a high-energy level, incident upon the alloy110, is used to deliver an electron current and thereby heat to thealloy 110. Use of the term “high-energy” is application-dependent andwill depend upon several device design parameters. A non-exclusive listof device design parameters influencing the term “high-energy” includesthe alloy material, the degree of conductivity change desired, thee-beam exposure time (corresponding to the time allotted to change theconductivity of the alloy), and the volume of the data cell within thealloy. A typical range of “high-energy” is 400–10,000 electron volts(eV) with an electron-beam current in the range of 10–3,000 nano-amps.

The alloy 110, shown in FIG. 1, may correspond to a data cell within anarray of data cells. The term “alloy” generally refers to a mixture ofat least two metals. Similar binary or n-ary resistance behaviors may becreated in organic or organo-metallic materials and these materials maybe applied in the same way at different power levels and cycle times fordata storage. It is well known that through thermal treatment, themicrostructures of certain alloys may be manipulated to produce stablephysical states with physical, chemical, and electrical properties thatvary from other stable physical states of the same alloy that have beenthrough different thermal treatment. Particularly relevant to thisinvention are the significant difference in electrical conductivitywhich exists between alloy states, and the thermal treatmentrequirements to change an alloy from one state to another. Chalcogenidealloys, preferred in this invention, are known to have stablecrystalline and amorphous states with significantly different electricalconductivities and adequate phase-reversal cycling capabilities.

A volume of chalcogenide alloy may be heated above its meltingtemperature and then cooled past its recrystallization temperatureeither to a primarily amorphous state (“amorphous state” or “amorphousphase”), or to a state having more crystallinity (“crystalline state” or“crystalline phase”), depending upon the cooling time during which thealloy remains above the recrystallization temperature (the “coolingprofile” of temperature versus time). To keep the material fromrecrystallizing during cooling and resulting in a crystalline phasestructure, the cooling rate must be faster than the crystal nucleationand growth rate for the particular material. Relatively fast coolingprofiles, therefore, are more likely to result in amorphous statestructures, and relatively slow cooling profiles are more likely toresult in crystalline state structures. Controlling the cooling profile,therefore, is a key variable for determining which logic state iswritten. As with other alloys, the electrical conductivity ofcrystalline phase chalcogenide alloy is significantly higher than thatof amorphous phase chalcogenide alloy due to the differences in physicalorder of the alloy material at the microstructural level and the factthat electrons travel more efficiently through ordered, or crystalline,microstructures. In terms of data bit memory state logic, the meltingmay be used to “reset” a data bit, and the cooling profile selected toeither result in a crystalline phase solid (either a “0” or a “1” in abinary example) or an amorphous phase solid (the binary complement ofthe crystalline phase logical representation). While multiplechalcogenide alloys with two or more stored phases accessible via e-beamirradiation are known, those comprising Germanium, Antimony, andTellurium are preferred, such as the alloy having the formula“Ge₂Sb₂Te₅”, with a melting temperature of 616 degrees Celsius and arecrystallization temperature of 142 degrees Celsius. With thispreferred alloy, an amorphous phase may be reached along a relativelyfast cooling profile by cooling past the recystallization temperaturewithin about two nanoseconds, while a crystalline phase may be reachedby a relatively slow cooling to the recrystallization temperature in aminimum of about two nanoseconds. Also preferred are alloys such asthose used in high-speed optical storage devices for “fast writing”,such as those comprising combinations of Germanium, Antimony, andTellurium, and combinations of Silver, Indium, Antimony, and Tellurium,wherein a crystalline phase of relatively high conductivity may bereached as a result of heating the fast-write material to about 85% ofthe melting temperature for the composition, followed by relatively slowcooling to enable heated material to nucleate relatively large,contiguous crystals based upon adjacent crystallites of the samematerial positioned around the boundary of the heated region. Suchfast-write alloys enable a faster change from amorphous phase tocrystalline phase, since the material need not be taken to the meltingtemperature before crystal nucleation. To reach an amorphous phase ofrelatively low conductivity with a fast-write material, heating to about110% of the melting temperature for the material is followed by coolingalong a relatively fast cooling profile, wherein the cooling rate isfaster than the crystal nucleation and growth rate for the particularmaterial to avoid the formation of a highly-ordered crystallinestructure. Therefore, as opposed to melting the alloy for a “reset” typeoperation, and cooling at various rates to proceed to either acrystalline phase, or an amorphous phase, as with conventionalchalcogenide alloys, fast-write alloys need not be melted unless anamorphous phase result is desired—since a crystalline phase may bereached by heating to about 85% of the melting temperature, followed bycooling along a relatively slow cooling profile. For example, afast-write alloy associated with cooling profiles similar to thosedescribed above in reference to the preferred Germanium, Antimony, andTellurium alloy, may be heated to about 85% of melting temperature andcooled to the recystallization temperature of the fast write alloy in aminimum of about two nanoseconds to provide a highly conductivecrystalline structure, while the same fast write material may be heatedto about 110% of melting temperature and cooled past itsrecrystallization temperature within about two nanoseconds to provide aless conductive amorphous structure.

E-beam irradiation is used both as a means for heating the alloy 110 tomelting temperature, and also as a means for controlling the coolingprofile of temperature versus time as the alloy is cooled subsequent tomelting. In one embodiment, for example, a reset and subsequentamorphous stored phase is achieved by e-beam irradiation to meltingfollowed by cooling without further e-beam irradiation, while a resetand subsequent crystalline stored phase is achieved by e-beamirradiation to melting followed by a decrease in e-beam irradiationalong a slower cooling profile. In other words, a crystalline storedphase is reached via a cooling profile wherein cooling is slowed bycontinued irradiation at a decreased rate. In one embodiment, a decreasein irradiation to slow cooling is achieved using power tapering, whereinthe voltage and/or current to the e-beam source is decreased. In anotherembodiment, intermittent exposure, or exposure tapering, is used todecrease irradiation during cooling. In another embodiment, a slowercooling profile is achieved by creating thermal mass in an adjacentstructure during cooling. Such thermal mass may be achieved using e-beamexposure. Hybrids of the aforementioned cooling profile controltechniques may also be utilized. For example, the cooling at a data celllocation may be controllably slowed by irradiating structuresimmediately surrounding the location to create thermal mass around thelocation, while the location itself also may be irradiated to a degreesufficient to contribute to slow cooling of the location. Manyvariations of exposure patterns may be employed in such hybridembodiments, including but not limited to spiral patterns, wherein aportion of the data cell location at the center of the spiral receivesmore focused irradiation than other portions, due to the focusing natureof a repeated spiral pattern and/or decreased radial spacing betweenspirals as they get closer to the portion of the data cell location atthe center of the spiral; stepped concentric circles used in a similarfashion as described for spirals, with the exception that the e-beam ispaused at each of a set of circular radii, the radii being equallyspaced or concentrated toward the center of curvature; and rectilinearscanning or rastering combined with focused irradiation exposure to atargeted location. Similarly, hybrid exposure patterning could be usedto emphasize exposure more upon the adjacent structures as opposed tothe data cell location itself. Exposure patterning may be combined withpower tapering and/or exposure tapering to provide further coolingprofile control. Cooling profiles for chalcogenide alloys such asGe₂Sb₂Te₅ are known in the art and used in devices such as CD-RW, whichuse laser light irradiation rather than e-beam irradiation to controlcooling.

Referring back to FIG. 1, in a data write phase, electron e-beam 120 (athigh-energy) creates a change in conductivity within the volume or layerof alloy 110. Reading the data is accomplished by introducing currentinto the alloy 110 via a low-energy e-beam, from either the write e-beamsource modulated to a lower energy or from an alternate e-beam at alower energy, than the energy level of the “high-energy” e-beam used forwriting the data. This low-energy e-beam used for reading may be lessthan approximately 100 electron volts (eV), and shall be of asufficiently small diameter so that it will not degrade the data bitmemory state of the alloy at the data bit location or any adjacent databit locations. In other words, the low-energy e-beam irradiation forreading data causes an excitation of the alloy 110 insufficient to heatthe alloy 110 above its recrystallization temperature.

The low-energy e-beam introduces an electron current into a data bitlocation corresponding with device 100 in FIG. 1. The conductor 112 andreference conductor 114 provide a differential conducting path, whichwill conduct a majority of the current indicated by 122 (the “bleed offcurrent”) when the alloy 110 is highly conductive. Alternatively, whenthe alloy 110 is highly resistive an increase will occur in the numberof the electrons that will propagate down through the alloy 110 andreference conductor 116 to the conductor 114. The conductor 114 and thereference conductor 116 provide a second differential conducting pathfor the current indicated by 124 (the “transmitted current”). Either oneor both of the bleed-off and transmitted currents may be used toassociate the state of conductivity within the alloy 110 with the memorystate of data stored in a bit. The e-beam 120 depicted represents eitherthe high-energy e-beam used to write the data or the low energy e-beamused to read the data.

Once the current has been steered, according to the state of the alloy110, many embodiments of the present invention may be used to sense,i.e. read the current. In one embodiment of the present invention, afixed impedance reference layer (typically with an impedance muchgreater than the low impedance memory state of the alloy storage layer)is attached to the alloy on the side opposite to the electron-beam. Theelectron-beam's current distribution is then measured differentiallybetween the conductor 112 and the conductor 114 using a current detector(not shown). The current will distribute differently based upon theconductivity of the alloy storage layer.

In another embodiment, the reference layer is replaced by using arelatively thick layer of the alloy where only the portion of this layerclosest to the electron-beam 120 is thermally modified to represent thememory state, while the remaining alloy material remains unchanged fromthe initial condition, thus acting as the reference layer cited in thepreviously-described embodiment.

In another embodiment of the present invention, the device 100 (FIG. 1)may be coupled with a light-emitting semiconductor P-N junction as shownin FIG. 2 to achieve a differential light emission device 200corresponding to the state of conductivity within the alloy 110. FIG. 2illustrates an alloy/LED configuration. With reference to FIG. 2, a P-Njunction 211 is placed beneath phase change material 110. The P-Njunction 211 may be created from a direct band semiconductor such asthose commonly made from III–V elements, e.g., GaAs, or other electronexcited light emitting structures. In one embodiment, doping is arrangedso that the N-type 210 layer of the P-N junction 211 is coupled with thealloy 110 allows for easy transport of the filtered electrons into theP-N junction 211. A very thin, conductive interlayer may be used tobackward bias the P-N junction from a high impedance source or toprovide lattice matching between the P-N junction and the alloy 110.Below a P-type 212 layer of the P-N junction, a conductor 214 is placedwhich may be a normal metal pad. The conductor 214 supplies holes to theP-N junction. The material used for the conductor 214 may be selected tooptimize the reflection of the light generated in the P-N junction. Anadvantage of using a direct-band semiconductor is that the recombinationof electrons and holes may create photons without requiring phononemission to conserve momentum. The thickness of the N-type 210 layer andthe P-type 212 layer must be sufficient to support the full transitionregion and optimally couple to external electromagnetic modes.

In another embodiment of the present invention, differential lightemission is achieved by the device 200 by varying the amplitude of thecurrent passed by the varying conductivity of the alloy 110 between thecross-linked and damaged states. In one conductivity state 218, thealloy 110 will conduct an increase in the number of electrons throughits thickness to the P-N junction resulting in a maximum emission oflight. The maximum emission of light may correspond to one memory statefor the data stored therein. The emitted light may be sensed byphoto-detector 224. In another conductivity state 216, a minimum amountof current is conducted through the alloy 110, instead the majority ofthe current is “bled off” via conductor 112, as previously describedresulting in a minimum emission of light from the P-N junction. Theminimum emission of light may correspond to a second memory state forthe data stored therein.

The alloy/P-N junction structure, shown at 200, need not be etched outfrom the surrounding material to allow photons to escape the P-Njunction 211. The layer (conductor 112) above the P-N junction may bemade very thin. Photons can penetrate more than a micron of metalconductor thickness and much further through other materials. Therefore,the conductor 112 may be on the order of a micron or less in thickness,thereby providing a sufficient electrically conductive path whileallowing photons to pass through the conductor 112. The device 200 actsas a tiny dot illuminator when irradiated with the read e-beam, causingthe P-N junction to emit light in one conductivity (memory) state whilethe P-N junction remains dark in another conductivity (memory) state.

An embodiment of the present invention is shown in FIG. 3 illustratingan alloy/P-N junction memory device. With reference to FIG. 3, one ormore alloy data elements are indicated at an alloy/LED 324 array. Avacuum shroud or enclosure 326 and an end cap 312 form a closedcontainer (a high vacuum environment) in which electron beam source 314emits e-beam 120, incident upon the alloy/LED 324 planar array. Controlelectronics 310 may be used in conjunction with the electron beam source314 as needed to control the e-beam source. The e-beam 120 may besteered by means of electron lens 316 and deflection electrodes 318.

The e-beam 120 may be used to write data to the alloy/LED 324 planararray, as previously described, as well as to read data written therein.Accordingly, the level of light generated by the alloy/P-N junction ofthe data bit is measured by a sensitive photo-detector 320, by methodswell known in the art. The vacuum shroud 326 may be made reflective onthe inside, thereby acting as an integrating sphere for the emittedphotons, which increases the signal-to-noise ratio for the measurementmade by the photo-detector 320. The output of the photo-detector 320 maybe amplified as required by photo amp 322. The e-beam 120 is steeredacross the phase change material/LED 324 array to read data stored inthe data bit locations corresponding to dots on the surface of alloy/LED324. Thus, writing and reading data is accomplished without themechanically articulated parts required by the hard disk drive, CD-ROMand the DVD. Using the teachings of the present invention, the seek timeto reach any block of data is on the order is tens of microseconds.

A read-after-write capability is provided during data bit writing by theconductivity state switching within the alloy 110. During data writingwith the high-energy e-beam, the impedance of the alloy 110 willchange—due to the induced state change, resulting in a sudden pulse oflight as electron current is passed or removed from the P-N junction.Sensing the pulse of light during the thermal decrease cycle willprovide a read-after-write capability similar to that provided by thesecond head in a tape drive or similar polarization shift effect in amagneto-optic disc drive.

In an alternative embodiment, the substrate (conductor 214 in FIG. 2)may be made sufficiently thin or transparent to allow photons to beemitted from the opposite side that e-beam 120 is incident upon phasechange material 110. This opposite side is indicated as 220 in FIG. 2and FIG. 3. The emitted light may be sensed from the lower side of thestructure as shown in FIG. 2 at photo-detector 224 a and in FIG. 3 atphoto-detector 320 a. The photo-detector 320 a may be configured withphoto-amp 322 a. This arrangement has the feature of removing theelectron optics from the photo-detector's field of view. One way toprovide a high vacuum environment to the lower side of vacuum shroud 326is to couple fixture 326 a with vacuum shroud 326 by mating as indicatedat 330 a and 330 b.

The e-beam 120 may be on the order of 20 nanometers in diameter. It ispossible to steer the e-beam 120 through an angle of approximately 20degrees, as will be explained in conjunction with FIG. 4. Very smalle-beam sources may be manufactured, made using silicon processes. Anexample of such a device is the electron micro-column made by theSensors Actuators and Microsystems Laboratory (SAMLAB), which is part ofthe Institute of Microtechnology, University of Neuchatel, located inSwitzerland. These very small e-beam sources (micro-column e-beamsources) may be produced in a form factor measuring approximately threemillimeters wide and approximately one millimeter high. In oneembodiment, using these parameters, a data storage device may be builtto store approximately a terabyte of data on a polymer/LED area ofseveral square centimeters.

The present invention may be incorporated into various memory devices.FIG. 4 shows an alternative embodiment of a memory device 400. Withreference to FIG. 4, a cylindrical container 412 is shown having a phasechange material/LED layer 324 and an electron beam source 314. Thee-beam source 314 may be steered through an angle 410 as shown. Thus,data may be written to an array having approximately 200 gigabytes ofdata in a device occupying a volume of approximately several cubicinches. These data storage devices may be configured in an array toachieve terabyte data storage capacities.

It is expected that many other shapes and configurations of data storagedevices are possible within the teachings of the present invention. Forexample, a cube may be configured (not shown) with phase change materialarrays lining the interior surfaces thereof. One or more electron beamsources may be configured within the cube, each facing an interiorsurface of the cube and being capable of writing and reading data storedin each phase change material array.

Thus, a novel solution to electron beam recording and sensing of databits is disclosed. Although the invention is described herein withreference to specific preferred embodiments, many modifications thereinwill readily occur to those of ordinary skill in the art. Accordingly,all such variations and modifications are included within the intendedscope of the invention as defined by the following claims.

1. A method of storing a data bit comprising: exposing a volume of alloyto an electron beam to melt the alloy; and cooling the volume of alloyalong a cooling profile associated with a stored phase and stored databit memory state, to a temperature below the recrystallizationtemperature for the volume of alloy, to achieve the stored phase,wherein cooling the volume of alloy along a cooling profile comprisesfurther exposing the volume of alloy to the electron beam to reduce arate of cooling of the volume of alloy.
 2. A method of storing a databit comprising: exposing a volume of alloy to an electron beam to meltthe alloy; and cooling the volume of alloy along a cooling profileassociated with a stored phase and stored data bit memory state, to atemperature below the recrystallization temperature for the volume ofalloy, to achieve the stored phase, wherein cooling the volume of alloyalong a cooling profile comprises exposing structures adjacent thevolume of alloy to the electron beam to reduce a rate of cooling of thevolume of alloy.
 3. A method of storing a data bit comprising: exposinga volume of alloy to an electron beam to melt the alloy; and cooling thevolume of alloy along a cooling profile associated with a stored phaseand stored data bit memory state, to a temperature below therecrystallization temperature for the volume of alloy, to achieve thestored phase, wherein cooling the volume of alloy along a coolingprofile comprises exposing both the volume of alloy and structuresadjacent the volume of alloy to the electron beam to reduce a rate ofcooling of the volume of alloy.
 4. The method of claim 1 wherein furtherexposing comprises exposing at a decreased level of irradiation usingpower tapering.
 5. The method of claim 1 wherein further exposingcomprises exposing at a decreased level of irradiation using exposuretapering.
 6. The method of claim 3 wherein exposing both the volume ofalloy and structures adjacent the volume of alloy to the electron beamcomprises exposure patterning.